Such plane or thin antennas have been in widespread use in numerous forms over the last 15 years or so. They have indeed become the norm in many fields, given their intrinsic qualities: low mass, volume, and manufacturing costs.
It is well known to the person skilled in the art that the simplest way of making radiating elements, i.e. microstrip tracks etched on a substrate, suffer from fundamental theoretical limitations, in particular with respect to bandwidth, directivity, and quality of radiation. The radiation is firstly highly asymmetrical in different section planes for an element that operates with linear polarization, and secondly it suffers from levels of cross-polarization that are often incompatible with the specifications of space missions.
From French patent application No. 93 03502 (corresponding to pending U.S. patent application Ser. No. 08/214,425) in the name of the Applicant, a multi-element system is known that enables the directivity of a printed antenna to be increased by using a sub-array made up of a multiplicity of elements that are mutually coupled electro-magnetically, and that are distributed over a surface that is plane or shaped.
It is also known from French application No. 89 11829 of Sep. 11, 1989 (corresponding to pending U.S. patent application Ser. No. 07/882,760) and in the name of the Applicant, to use metal cavities to increase the bandwidth of a printed radiating element. That configuration also makes it possible to control radiation and inter-element coupling in an array made up of such elements.
Compared with conventional solutions such as horns or dipoles, solutions that make use of printed elements have the advantages of lower weight and bulk, however they also have lower performance with respect to various parameters of antenna operation. In particular, it turns out to be difficult to obtain simultaneously acceptable bandwidth with determined directivity and polarization purity compatible with telecommunications applications.
At present, printed radiating elements have directivities that lie conventionally in the range about 5 dBi to about 10 dBi as a function of the geometrical characteristics of the antenna (thickness of substrate, dimensions of radiating patches and of cavities if any) and of the materials used (dielectric constant of the substrates).
These directivity values are also intrinsically related to the resonant dimensions (about half the wavelength of the guided wave) of that type of radiating element which limit their radiating areas and consequently the maximum directivity that is accessible.
The conventional solution for obtaining greater directivity from a printed antenna is to put the radiating elements in an array. That generally leads to designing a feeder network for generating the feed relationships that are necessary for forming the desired radiation characteristics. Such feeder systems are necessary in particular for apodization of the relationship whereby the radiating aperture is fed, thus making it possible to avoid the appearance of secondary lobes, which are often undesirable in antenna systems for radar or for telecommunications.
The design of such feeder systems presents a certain number of problems, which are described in greater detail in French application No. 93 03502. Of those problems, the following may be mentioned briefly:
1) The complexity of such a system increases with the number of elements to be fed; complexity is even greater for an antenna operating with circular polarization.
2) The discontinuity or discretization of the apodization, due to the radiating area being sampled by discrete elementary radiating elements.
3) Coupling between the elements is difficult to take into account, and it is generally considered as being a phenomenon that tends to degrade the performance of the antenna.
4) Connections are complex, thereby tending to reduce the reliability of the antenna.
5) Losses in the energy distributor may be considerable, thereby hindering the use of such a solution at very high frequencies or for passive antennas having several tens of elements, since resistive losses become unacceptable under such circumstances.
Those problems are known to the person skilled in the art, numerous attempts have been made to ameliorate them, and they constitute the subject matter of many publications, with full treatment being given in "Handbook of Microstrip Antennas" by J. R. James, P. S. Hall, and C. Wood, appearing in IEE Electromagnetic Waves Series, No. 12, published by P. Perigrinus Ltd., Stevanage, UK. That publication forms an integral part of the present application for its description of the prior art.
However, the solutions proposed in the prior art imply making compromises, since other performance criteria of the antenna are limited as a counterpart to improvements in directivity. For example, the article by R. Q. Lee, R. Acosta, and K. F. Lee: "Radiation characteristics of microstrip arrays with parasitic elements" published in Electronics Letters, Vol. 23, pp. 835-837 (1987) describes a device that gives 11 dBi of directivity, but with a bandwidth of less than 1%, a total substrate thickness that is very high being of the order of 0.4 .lambda., and all that without any control of polarization or of symmetry in the radiation pattern.
Two other solutions propose enlarging the radiating area to improve directivity, either by coupling radiating elements in the same plane as the exciting patch, or else by fragmenting the upper resonating patch in a structure that has two superposed patches. The first solution is described by R. Q. Lee and K. F. Lee in "Experimental study of the two-layer electromagnetically coupled rectangular patch antenna": IEEE Transactions on Antennas and Propagation (1990), Vol. AP-38, No. 8, pp. 1298-1302; while the second solution is described in French patent application No. 93 03502.
The directivity improvements presented by those known solutions nevertheless remain modest because of insufficient coupling in the first-mentioned case, and because of a radiating area that is still insufficient in the second solution.